1. Field of the Invention
The present invention relates to a pneumatic/hydraulic accumulator "heat engine", which may be used in hydraulic powertrains for vehicles, but is further useful with any source of heat to produce shaft work.
2. The Prior Art
Hydraulic accumulators typically operate over pressure ranges between about 1,000 psi and 5,000 psi or greater. Most of this pressure change is the result of the increase or decrease in the volume of the fluid from the liquid side of the accumulator, which alternately compresses and expands the gas side, thereby alternately storing and releasing energy. Heating and cooling of the gas side also occurs as a natural consequence of rapid compression and expansion, and has an additional effect on pressure. Reading FIG. 1 from the right, curve A shows the rate of pressure increase as it would have occurred had there been no temperature increase, i.e. an isothermal compression. Curve B shows the actual rate of pressure increase due to both volume compression and natural heating during a rapid compression. Since the area under the curves represents work performed, it can be seen that isothermal compression requires less input energy than rapid natural compression, although the pressure achieved, and thus the amount of energy stored, is smaller. However, before the gas in the vessel is expanded, any heat naturally generated by compression may be lost through the walls of the device. Since this heat was actually provided by the work performed in compressing the gas, it is lost work.
As the accumulator discharges, the pressure drops off very quickly as a result of (a) volume expansion, and (b) a drop in temperature, the reverse of the normal compression process. It would be more desirable to maintain the pressure as high as possible during expansion.
Conventional accumulators offer no means to improve the shape of the pressure curve because they cannot control the temperature change in the charge gas. The temperature change cannot be purposely controlled or harnessed in any way that would act to improve the specific energy storage capacity or other characteristics of the device. Instead, it is tolerated as a natural outcome of accumulator design and is worked around or ignored. For example, conventional accumulators do not allow isothermal compression and expansion in normal use, even though an isothermal process might lead to advantages in efficiency, specific energy storage capacity, pressure curve, and volume sizing considerations.
There have been investigations into the use of insulating foam materials in the gas side of an accumulator to contain heat and prevent its loss to the environment. While this technique has been shown to increase the efficiency of accumulators to dramatic levels (nearly 100%), it does not significantly increase the specific energy storage capacity of the accumulator, and does not significantly affect the shape of the pressure curve.